1. Field of the Invention
The present invention relates to an apparatus and a method for transmitting ultrasonic waves. More particularly, it relates to an ultrasonic transmitter used for an autofocusing device built in an image producing apparatus.
2. Description of the Prior Arts
Conventional auto-focusing apparatus utilizing ultrasonic waves for distance measurements, in general, comprises a means for transmitting ultrasonic waves for a specified short time period and receiving reflected ultrasonic waves, a processing means for generating a control signal responding to a time for the ultrasonic waves traveling from and to the transmitting/receiving means, and further an auto-focusing means for moving lens responding to the generated control signal from the processing means. When such auto-focusing apparatus is employed for monitoring positions of continuously moving objects, for example, in a video camera, it is required for the auto-focusing apparatus to continuously control lens focusing positions base on continuous distance measurements. The following descriptions will be given to video cameras as one example of image producing apparatus provided with auto-focusing apparatus.
Necessity of generation of a strong ultrasonic sound energy for the ultrasonic wave transmitting, sufficient for a distance to be measured, usually compels us to employ a step-up transformer for generation of a high voltage applied to an ultrasonic wave transducer in the auto-focusing apparaus. But, employment of such step-up transformer is liable to cause undesirable voltage variations of a power source contained in the video camera, since an impedance for a load circuit for the power source decreases at each initial moment of the step-up transformer's step-up operation for generating a high voltage to be applied to the ultrasonic wave transducer, and since this makes energy consumption for the power source intermittently large. The above voltage variations cause problems related with image noises during image reproduction.
Distance measurement operations utilizing ultrasonic waves are completed only after receiving reflected ultrasonic waves following transmission of the waves. That means, in order to enable repeated distance measuring required, for example, for the video camera, it becomes necessary to send out ultrasonic waves at an appropriate repetition time interval during the auto-focusing operation for the video camera. But such repeated transmission of the ultrasonic waves causes the above-mentioned image noise problems.